1. Field of the Invention
The present invention relates to automotive-type rectifier assemblies used to convert polyphase alternating current to direct current using silicon semiconductor diodes. More particularly, the invention deals with the high power and under-hood temperatures required by modem day automobiles and the problems associated with overheated and stressed semiconductors. Further, the invention relates to the complex manufacturing and operational problems associated with the sensitivity of semiconductors to the heat, stress and mechanical forces, as well as a method of manufacturing semiconductors, thus eliminating these problems.
2. Description of the Related Art
Much work has been done in the past to decrease manufacturing costs, air pollution and weight, thus decreasing the alternator""s size. Paradoxically, the electrical energy requirements increased. Compact alternators operating under these conditions cannot dissipate heat out of the rectifier bridge fast enough to prevent semiconductor failures. This is particularly true during the summer months when the ambient temperatures are quite high, providing minimal heat transfer and a higher alternator failure rate.
The present invention addresses these problems, especially the replacement expenses for the remanufacturing industry, the general public, and also the wasted energy.
The present invention is further concerned with state of the art manufacturing, because of the chip""s extreme sensitivity to the thermal and mechanical stresses associated with it. The invention described herein avoids the stresses by never exceeding the semiconductor""s technical and handling specifications. The new and novel invention decreases the horrendous alternator failure and return rate caused by the semiconductors. This is a general nuisance for the public, costing millions of dollars, for towing, repairs and replacement alternators, wasting energy and generating unnecessary air pollution.
The prior art teaches that polyphase alternating current can be converted to direct current by conducting it through six or more semiconductors pressed into prepunched holes in the rectifier heat sinks used in the automobiles"" alternators, as illustrated by U.S. Pat. Nos. 5,043,614 and 5,712,517, which are incorporated by reference herein.
The above type inventions are complex to manufacture because the semiconductor chip""s extreme sensitivity to the heat, stress and mechanical forces applied during the manufacturing of rectifier bridges causes production failures and causes premature catastrophic failures during the vehicle operation.
Historical data collected from returned alternators under warranty indicate their semiconductors have a very high mortality rate. Historically, 74% fail within the first month""s operation, 15% during the next 2 months, 4% after 3 months and 7% during the next 4 months. This failure rate can be directly correlated to manufacturing damage.
This is especially so because silicon semiconductor chips are extremely fragile, being only 0.180xc3x970.180xc3x970.007 inches thick (the thickness of three human hairs), making them so sensitive to the pressure, stress and heat, which are all required to press them into the state of art rectifier bridges.
When chips this thin are soldered inside the bottom of a thick copper cylinder and then filled with an epoxy, a solid mass is formed. Being solid, the chip has no clearance for thermal or stress relief, all transmitted into them during soldering, installation and the alternator operation. Therefore, if a prestressed, overheated chip does not fail during the manufacturing and testing, if damaged, it will likely fail in the alternator, exposed to the constant heating, cooling and load changes. Thus, the chips have a high morality rate.
Most press-fit rectifier bridges incorporate two heat sinks, one usually being a positive aluminum heat sink, the other being the negative die-case aluminum rear alternator housing. They are separated from each other by a heat conductive, electrical insulator gasket or a ceramic coating on the housing, which is a very expensive process. Both of the heat sinks have prepunched or machined holes for at least three press-fit semiconductors. The cathodes of the first three are pressed into the positive heat sink, and the anodes of the other three are pressed into the negative heat sink. At least three stator terminals, stamped out of copper, are usually encapsulated into a plastic assembly. Each has two locating holes to affix a positive set and a negative set of semiconductor leads in series, along with a slot, which connects a set of stator field leads between them.
The first set of stator terminals connects the anode lead from the first positive semiconductor to the cathode of the first negative semiconductor, thereby forming a set of series connected semiconductors between the two heat sinks. The leads from the alternator""s polyphase field winding is affixed into the slot, which connects it between the series diodes, as described above. The next two sets of semiconductors are similarly connected to the second and third sets field windings, completing the polyphase field circuits. A lead for the voltage regulator is also affixed between the third set of semiconductors. A stud is affixed to the positive heat sink to complete the B+ circuit to the positive post of the battery. Likewise, the negative heat sink completes the charging system""s circuit to the negative post when the alternator is installed.
Prior to assembling the rectifier bridge, a thin coating of thermally conductive grease is applied onto the surfaces of the heat sinks in an attempt to increase the heat transfer out of the semiconductors affixed to the positive heat sinks, through the insulator gasket and into the aluminum die-cast rear housing, into the cooling air flow. However, the semiconductor chips are affixed deep into thick copper cylinders, surrounded by an epoxy filler, thus delaying the heat transfer out of them and causing even more chip failures.
The automotive industry being very competitive, continually requires cost efficiency and less failures.
State of the art rectifier bridges using pressed-in semiconductors are a continuous nuisance and problem for the general public, requiring expensive service calls, towing and repairs. It is the object of the present invention to eliminate this costly nuisance by eliminating these failure problems, further advancing the state of the art by its unique design, creating a method of manufacturing the bridge rectifier without stressing or damaging the sensitive semiconductor chips inside the diodes. It also instantly dissipates their heat out directly into the outside cooling air, being affixed directly onto copper assemblies and stator terminals, which are in direct contact with the cooling air flow. The thermal transfer (compound) gasket also transfers the heat directly into the housing and to the outside cooling air quicker.
The invention is so unique that thousands of dollars can be saved by using it to remanufacture existing rectifier bridges discussed above. This is accomplished by machining off the upper section of the press-fit copper cylinders (removing the plastic and chip section), which is then pressed out of the sinks. The blank copper slugs are placed into hydraulic press fixtures and pressed into solid copper cavity nest assemblies, which will then nest the pan and/or button type semiconductors.
The newly formed copper assemblies are then pressed back into the existing heat sinks, without chips. The reused copper not only decreases the manufacturing cost but also saves our precious earth elements and the energy required to manufacture new ones. The present invention is especially unique because it decreases the horrendous failure problems associated with the present state of the art rectifier bridges, which is the primary cause of alternator failures. However, the known bridges are remanufactured, only to fail again, because the remanufacturers have no choice but to press in the same type semiconductors that failed (which will fail again).
The invention provides a bridge rectifier assembly which is manufactured in a new and simplified production manner, using stamped and molded parts in a new and novel form, so that pan or button type semiconductor diodes can replace the present press fit type diodes.
Typically, the diodes are affixed onto preformed copper cavity nest assemblies, which can be formed utilizing sintered copper slugs or punched out solid copper strips and/or by reusing the remanufactured copper diode cylinders and hydraulically forming into cavity nest assemblies which are then pressed into the existing heat sink holes, without the sensitive semiconductors affixed onto them. They can also be assembled using the two copper heat sinks, with the nest cavities formed into them, in accordance with U.S. Pat. No. 5,659,212, which is incorporated by reference herein. The commercial state of the art press-fit type semiconductors are not required with the present invention.
Stator terminals are also stamped out of copper, having two holes for the semiconductor diode leads, but without slots for the stator field coils, as used on the state of the art rectifiers. The present invention has preformed bends, which can be aligned with the stator leads, then crimped after placing special type copper cups over them, eliminating the possible hazardous soldering process, which is also very time consuming and costly, and which also exposes the chips to further damage.
The present invention also utilizes a new and unique insulator manufactured out of a type of melamine formaldehyde, molded, glass-fiber filler, having most of the inside area cut out, so that a heat sink compound, such as Dow Corning 340, can be filled into the cut out area for better heat transfer, and also having a very high compression strength rating. This invention can also be used as a high strength heat transfer gasket for mounting silicon controlled rectifiers and other types of transistors and/or components requiring high thermal transfer and torque down strength.
A preferred embodiment of the present invention utilizes at least six spring-loaded diodes affixed onto two copper heat sinks, which operate cooler and more efficiently as previously described here and also in my existing U.S. Pat. No. 5,659,212, which is incorporated by reference herein.
The heat sinks with the semiconductors affixed onto them are separated by a thermally conductive, electrical insulating type gasket, with only a peripheral border allowing the center cutout area to be filled with a thermal heat transfer compound as described previously above.
The cathodes from three semiconductors form the positive heat sink and the anodes from three others form the negative sink. A lead connects the anode of the first positive semiconductor to the cathode of the first negative semiconductor, thereby forming a set of series connected semiconductors between the two heat sinks. A lead from the alternator""s polyphase field winding is also affixed between the two diodes. The next two sets of semiconductors are similarly connected, completing the polyphase circuits. A lead for the voltage regulator and electric choke (if used) is also affixed between the third set of semiconductors. A B+ stud connected to the positive heat sink completes the circuit to the positive post of the battery. The negative heat sink completes the charging system""s circuit to the negative battery post when the alternator is installed.
Typically, the stator terminals are riveted to the heat sinks and insulated from them by nylon bushings. A spring compressed between the stator terminal and the semiconductor locates and applies pressure to affix the semiconductor""s contact against the copper heat sink nest, assuring an excellent electrical and thermal contact; however, they can be affixed to the heat sinks without the compressed springs, if required for cost savings. The crimp-type terminals also decrease the rebuilder""s manufacturing cost, eliminating potential damage to the semiconductor.
One aspect of the present invention is the manufacturing of bridge rectifiers using compression springs, stamped out parts and terminals, so that the pan or button type diodes can be used successively, whereby the expensive computerized presses are not required (which are used in prior processes to attempt to prevent fractured and stressed chips).
State of the art rectifier bridges primarily use press-fit semiconductors because prior to the present invention, the diodes tended to become unsoldered during momentary power surges, floating off the heat sink, causing them to lose contact and causing the alternator to fail. The paradox is that using a higher melting solder causes the semiconductor chips inside the diodes to be possibly damaged and fail, whereas the press-fit diodes do not require soldering. However, the present invention allows any semiconductor diode now to be soldered and gently held in place by the springs, even if the solder remelts. The manufacturer""s compression, torque and thermal shock specifications are never exceeded. Their life is also extended because they operate cooler. The use of a thermal transfer gasket also eliminates the ceramic flame spraying and/or epoxy dipping processes, required by the state of the art bridge rectifiers, to insulate the heat sinks from each other and to transfer the heat out of them, both being very expensive operations and not as efficient as with my gasket.
The longer life and decreased productive losses also decreases air pollution by requiring less manufacturing energy, thus saving the environment.